This invention relates to a heavy duty manual transmission, and more particularly, the invention relates to a shift shaft switch for the transmission which senses the location of the shift shaft.
Heavy duty tractor trailer transmissions have become more complex in recent years. As class 8 transmissions have evolved from pneumatic to electric controls, electronic sensors have become necessary to provide information to the transmission controller. Some of these sensors affect the driver while in the act of engaging or disengaging gears.
Heavy duty manual transmissions typically have shift shafts supported by a transmission housing. Shift forks are typically connected to the shift shafts and move the clutch collars into and out of engagement with the gears as the shift shafts are rotated and translated in response to the operator manipulating the shift lever. The sensors coact with the shift shaft to sense the position of the shift shaft which corresponds to a particular gear position so this information may be sent to the controller for control of the transmission or other aspects of vehicle operation.
In the past, the shift shaft has included a single annular groove that corresponds to the neutral gear position of the transmission. A pin is biased into the groove by a sensor that has an integral spring. The sensor detects the location of the pin along the shift shaft. When the pin is seated in the annular groove, the sensor detects that the transmission is in a particular position or state, such as the neutral gear position. Undesirably, the vehicle operator must overcome the force of the spring as the pin travels up the annular groove when shifting out of the neutral position and into engagement with a gear. This may increase the shift effort or reduce the shift feel. Therefore, what is needed is an improved switch assembly and shift shaft arrangement that does not adversely affect shift effort or shift feel.
The present invention provides a transmission gear position sensing assembly. The assembly includes a transmission housing with a bore having a shift shaft, such as a shift shaft disposed therein. The shift shaft has an axis about which the shaft translates as the transmission is shifted between gear or synchronizer positions. The housing includes a hole with a switch assembly received in the hole transverse to the shift shaft axis. The shift shaft includes a profile aligned with the switch assembly. The profile has first, second, and third surfaces respectively spaced radially from the axis a first, second and third distance. The third surface is arranged between the first and second surfaces. The third distance is larger than the first and the second distances, and the third surface corresponds to a neutral gear position. The switch assembly includes a member biased into engagement with the profile by a spring. The switch assembly senses the gear position by the corresponding location along the profile. Since the third surface, which corresponds to the neutral position, is raised from the other surfaces, the shift effort is reduced as the vehicle operator shifts from the neutral position to one of the other gear positions. In this manner, the switch assembly and switch shaft configuration does not adversely effect the shift effort or shift feel.